Once any item of information is placed on your hard drive, it remains there forever unless one of two things occur. Deleting any information merely allows for that space on the hard drive to be written over, but the information is just buried under future layers of information and is retrievable by a technician. Military type over writing such as the free "DBAN" program ( Darrik's Boot and Nuke ), can be run several times, but completely wipes the hard drive of all information including the operating system, which must be re-installed. The second method involves removing the hard drive and physically destroying it and replacing it with a new hard drive and re-installing the operating system.